Hi,
I would see your GP as well as a nutritionist. I am in the same boat and had to have various medical tests to rule out medical conditions. Everything was ruled out. I then had to keep a food diary to give my nutritionist an idea of what I was eating in a day, but that was all normal too.
Now I am currently working on building my muscle tone to fill myself out a bit and give me the extra weight. Nothing I did or could have done would have made me put weight on and maintain it, so this is my only option.
Don't worry about turning into a body builder :lol: that takes a lot of work and protein and special body building exercises. I do mainly dancing and calisthenics to build up my abdominal, gluteal (buttock) and quadricep (thigh) muscles.
The thing I found the hardest were the comments other people made about my lack of weight, and the fact it seems to be socially acceptable to make comments about being too skinny but not too fat. It hurts.
Good luck!